Alison Campbell considers the evidence for the efficacy of parachutes.
Recently a teacher sent me a paper titled: ‘Parachute use to prevent death and major trauma related to gravitational challenge: systematic review of randomised controlled trials‘ (Smith and Pell, 2003, BMJ 327: 1459-1460). I have to say I chuckled when I read this – a common charge levelled against current medical practice by the alternative health lobby is that many medical techniques haven’t been subjected to randomised controlled trials (with the corollary that it’s thus unfair to demand evidence from such trials on alternative practices).
The authors state they conducted a literature search of some of the major science sources, using the search words ‘parachute’ and ‘trial’. However (and unsurprisingly), they found no randomised controlled trials (RCTs) of parachute use. Smith and Pell begin their discussion with the following inspired statement:
“It is a truth universally acknowledged that a medical intervention justified by observational data must be in want of verification through a randomised controlled trial.”
Many medical interventions probably fall into this category – for example, I doubt that surgery for severe appendicitis has ever been subjected to such a trial. That’s not to say that, where appropriate (and in the case of appendicitis it almost certainly isn’t!) such trials shouldn’t be performed. As Smith and Pell point out, hormone therapy for post-menopausal women seemed – on the basis of observational studies – to convey a number of health benefits. But RCTs showed that hormone replacement therapy actually increased the risk of ischaemic heart disease.
As the authors say, RCTs avoid a major weakness of observational studies: that of bias (eg selection bias and reporting bias). They note that individuals jumping from aircraft without the help of a parachute are likely to have a high prevalence of pre-existing psychiatric morbidity (ie they are probably not in their right minds when they jump. You have got to love this paper!). So any study of parachute use could well be subject to selection bias, in that those using them are likely to have fewer psychiatric problems than those who don’t. Smith and Pell also put forward the possibility that enforced parachute use is simply a case of mass medicalisation of the population by out-of-control doctors – or worse, by evil multinational corporations. (These are, of course, charges frequently levelled at the medical world, eg by those who are against interventions such as vaccination.)
This little gem of a paper contains some valuable lessons on the nature of science (and more particularly, science-based medicine). And it should be read by anyone who doubts that scientists have both creativity and a good sense of humour.